1. Field of the Invention
The invention relates to a vibrating device for the processing of an optical beam, that is to say to a device including an optical member and a frame supporting the device and means of impressing upon the optical member a vibratory motien relative to the frame.
By "optical member" is understood here any element intended for imposing upon a beam of electromagnetic energy an optical process such as a reflection, a refraction, a diffraction, a modulation of phase or of amplitude, etc. As examples of optical members which may be subjected to a vibratory motion of rotation or translation relative to the frame of an optical apparatus or an element of an optical bench, may be mentioned the movable gratings or the reflecting mirrors of infrared grating spectrometers, the networks employed in holographic techniques, the models for photoelasticimetry, etc.
The vibratory motion is obtained either by means of a positive mechanical control (including, for example, an electric driving motor and a connection by connecting-rod or by cam), or by means of a mechanical oscillator supporting the optical member and excited at its fundamental resonant frequency. This second solution is generally preferred, especially for reasons of bulk and efficiency.
However a vibrating resonator device connected to a frame excites vibrations in the frame. This disadvantage becomes very disturbing when the apparatus or the bench is intended for tests or measurements which demand a high level of resolution and/or great sensitivity. The solution to the problem which one encounters with such vibrations is to overdimension the frame in order to increase its rigidity and its mass. This is acceptable for apparatus or benches employed at a fixed station but it is not acceptable apparatus mounted in aircraft or satellites, in which the mass of the airborne material must be restricted.
2. Description of the Prior Art
A solution has already been proposed in French Pat. No. 2 116 089, which in certain cases enables the excitation of the frame to be reduced. It consists of a torsion-bar resonator device intended for making a mirror oscillate in rotation, and connecting the ends of the torsion-bar to the frame by means of resilient pivots, that serve as flexible members the stiffness of which is very much less in torsion than the stiffness of the bar. This particular solution is not very satisfactory (especially for an airborne apparatus) because the portion of the frame connected directly to the resonator is subjected to a torque which cannot be neglected and which comes from the reaction of the excitation system. The vibrations transmitted to the frame of the apparatus and to the other members which the frame supports bring about a dissipation of energy which can only be borrowed from the excitation member of the resonator. The factor of resonance Q of the vibrating system becomes altered and this factor depends upon the internal damping of the structure consisting of the apparatus and its support. Now this damping itself varies with the ambient conditions (temperature, pressure, mechanical stresses due especially to inertia in the case of acceleration, variable gravity, etc.). The parameters which characterize the vibratory motion (especially the frequency and the amplitude) are no longer constant.
It has also been proposed to reduce the parasitic vibrations generated on the support of a device endowed with an oscillating motion by providing a frictionless dynamic damper consisting of an oscillating system comprising a mass supported by resilient means judiciously dimensioned and fixed to the support. The parasitic vibrations excite the dynamic damper which oscillates and produces a force of reaction which is at any moment equal and opposite to the disturbing force. The dynamic dampers which are suitable for industrial applications in which a considerable residual level of vibration is accepted, but such industrial dampers do not have a sufficient efficiency to be effective when installed to compensate for vibration optical apparatus.
The invention enables all of these disadvantages in the prior art to be avoided because it provides a novel vibrating device which does not react upon the portion of the frame to which it is connected. More precisely, this device of the present invention includes sensitive and effective compensation means which apply constantly to the said portion of the frame a reaction which cancels out the reaction applied by the resonator.